Cross-posted from Coyote Blog
The EPA has done the fuel economy rating for the all-electric Nissan Leaf. I see two major problems with it, but first, here is the window sticker, from this article
Problem #1: Greenhouse gas estimate is a total crock. Zero?
The Greenhouse gas rating, in the bottom right corner, is that the car produces ZERO greenhouse gasses. While I suppose this is technically true, it is wildly misleading. In almost every case, the production of the electricity to charge the car does create greenhouse gasses. One might argue the answer is zero in the Pacific Northwest where most power is hydro, but even in heavy hydro/nuclear areas, the incremental marginal demand is typically picked up by natural gas turbines. And in the Midwest, the Leaf will basically be coal powered, and studies have shown it to create potentially more CO2 than burning gasoline. I understand that this metric is hard, because it depends on where you are and even what time of day you charge the car, but the EPA in all this complexity chose to use the one number – zero – that is least likely to be the correct answer.
Problems #2: Apples and oranges comparison of electricity and gasoline.
To understand the problem, look at the methodology:
So, how does the EPA calculate mpg for an electric car? Nissan’s presser says the EPA uses a formula where 33.7 kWhs are equivalent to one gallon of gasoline energy
To get 33.7 kWhs to one gallon, they have basically done a conversion through BTUs — ie 1 KWh = 3412 BTU and one gallon of gasoline releases 115,000 BTU of energy in combustion.
Am I the only one that sees the problem? They are comparing apples and oranges. The gasoline number is a potential energy number — which given inefficiencies (not to mention the second law of thermodynamics) we can never fully capture as useful work out of the fuel. They are measuring the potential energy in the gasoline before we start to try to convert it to a useful form. However, with electricity, they are measuring the energy after we have already done much of this conversion and suffered most of the losses.
They are therefore giving the electric vehicle a huge break. When we measure mpg on a traditional car, the efficiency takes a hit due to conversion efficiencies and heat losses in combustion. The same thing happens when we generate electricity, but the electric car in this measurement is not being saddled with these losses while the traditional car does have to bear these costs. Measuring how efficient the Leaf is at using electricity from an electric outlet is roughly equivalent to measuring how efficient my car is at using the energy in the drive shaft.
An apples to apples comparison would compare the traditional car’s MPG with the Leaf’s miles per gallon of gasoline (or gasoline equivalent) that would have to be burned to generate the electricity it uses. Even if a power plant were operating at 50% efficiency (which I think is actually high and ignores transmission losses) this reduces the Leaf’s MPG down to 50, which is good but in line with several very efficient traditional cars.